Time Machine - Frequently Asked Questions
20. Once my Mac is backed-up, can I delete some stuff to save space?
Time Machine - Frequently Asked Questions
20. Once my Mac is backed-up, can I delete some stuff to save space?
NO, NO, NO! That's a terrible idea !
First, the whole point of a good backup strategy is to have (at least) two copies of everything important, in (at least) two different places. If you delete the originals, you no longer have backups! When your Time Machine drive fails (and all disk drives fail, sooner or later), you risk losing your only remaining copy.
Second, Time Machine will, sooner or later, delete the backup copies of anything that's no longer on your system. The timing varies, depending on how long things were on your system before being deleted, how often backups were run, and how much space is on your Time Machine drive: it may be as long as your oldest backup, or as short as 24 hours. See question #12 for details.
If you're nearly out of room on your internal HD, your best bet may be to get an external HD for the "overflow" of stuff that won't fit on your Mac. Move it there by dragging via the Finder (opening one Finder window to your internal HD and a second one to the external may be easiest), then once you're sure everything was copied ok, delete them from your internal HD.
Be sure to format the drive for a Mac. See #1 in Using Disk Utility if you're not sure how to do that.
Then let Time Machine back up both your Mac and the external HD. Externals are excluded by default, so you'll have to tell Time Machine to include it by removing it from the exclusion list in Time Machine Preferences > Options.
See question #32 for details and considerations of backing-up multiple drives.